[Mister Pontiac]

About 10 years ago, I redid my 67 from the firewall forward. Mine is a Pontiac plant car as well.

As I was stripping the old rattle can paint off of the firewall, I found the factory paint and grease pencil markings underneath. The paint was some sort of undercoating type of material as the chemical stripper would not touch it while it peeled off the rattle can paint with ease. Mine was factory matte black (60%?) from the cowl ledge down.

Here's a picture of it when I was done stripping and cleaning the firewall. Like you, I 'refreshed' the grease pencil markings, but I did it directly over the original marks.

Hope that helps.